ORGANIC LETTERS 2005 Elysiapyrones from Elysia diomedea. Vol. 7, No. 3 Do Such Metabolites Evidence an 415-418 Enzymatically Assisted Electrocyclization Cascade for the Biosynthesis of Their Bicyclo[4.2.0]octane Core? Mercedes Cueto,† Luis D’Croz,‡,§ Juan L. Mate´,‡,⊥ Aurelio San-Martı´n,| and Jose´ Darias*,† Instituto de Productos Naturales y Agrobiologı´a del CSIC, AVda. Astrofı´sico F. Sa´nchez, 3, 38206 La Laguna, Tenerife, Spain, Smithsonian Tropical Research Institute (STRI), P.O. Box 2072, Balboa, Panama, Departamento de Biologı´a Marina y Limnologı´a, Estafeta UniVersitaria, UniVersidad de Panama´, Panama, DiVision of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, UniVersity of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, and Departamento de Quı´mica, Facultad de Ciencias, UniVersidad de Chile, Santiago de Chile, Chile [email protected] Received November 3, 2004 ABSTRACT Biogenetically interesting polypropionate-derived metabolites 1 and 2, featuring an unprecedented skeleton, have been isolated from the sea slug Elysia diomedea. Their enantiomeric character indicates that the current spontaneous electrocyclization cascade biogenetic hypothesis for the bicyclo[4.2.0]octane core must be enzymatically aided. These compounds are isomeric with the 15-nor-9,10-deoxytridachione/15- norphotodeoxytridachione pair of metabolites and encourage speculation about their biosynthetic relationship. The sacoglossan Elysia () Tridachiella) diomedea (Bergh) their own tissues.1 While the plastids remain photosyntheti- (Mollusca, Opisthobranchia, Sacoglossa) belongs to one of cally active they provide an energy supply, and in some the few metazoan groups that retain chloroplasts harvested cases, the released organic carbon can totally sustain the sea from siphonous marine algae to carry out photosynthesis in slugs for several months in the absence of an algal food source,2 giving rise to the issue of whether lateral gene † Instituto de Productos Naturales y Agrobiologı´a del CSIC. transfer has occurred between eukaryotic alga and animal ‡ Smithsonian Tropical Research Institute. § Universidad de Panama. ⊥ University of Miami. (1) Trench, R. K.; Greene, R. W.; Bystrom, B. G. J. Cell. Biol. 1969, | Universidad de Chile. 42, 404-417. 10.1021/ol0477428 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/13/2005 nucleus.3 Faced with life lacking a protective shell, some core that features an unprecedented carbon skeleton, ely- sea slugs evolve chemical defense mechanisms against siapyrone. These compounds were isolated together with the predators based on diet-derived and, mostly, endogenous known metabolites tridachione, 9,10-deoxytridachione,5 iso- feeding deterrents4 produced by joining several propionate 9,10-deoxytridachione,7 and 15-norphotodeoxytridachione.9 units, two of which are involved in the formation of a γ-pyrone ring that has become a common feature of the Elysiidae family. Naturally occurring metabolites with a backbone of six, seven, or eight propionate units, sometimes elongated by a two-carbon appendage (acetate chain starter unit), have been isolated from the genus Elysia and can be grouped within the structural types represented by 9,10-deoxytridachione,5 photodeoxytridachione,6,7 or tridachiahydropyrone.8 More- over, pairs of cyclohexadiene/bicyclo[3.1.0]hexane isomers R 25 containing six, seven, or eight propionate units have been Elysiapyrone A, 1, was isolated as a colorless oil, [ ]D )+37° (c 0.08, CHCl3). The molecular formula suggested isolated, for instance: 15-nor-9,10-deoxytridachione/15- + norphotodeoxytridachione7,9 (six), 9,10-deoxytridachione/ by the HREIMS, [M] 360.1883 (calcd for C21H28O5, 13 photodeoxytridachione5-7 (seven), and elysione/crispatene5,10 360.1936), indicates eight degrees of unsaturation. The C (eight propionate units). This isomerization has been proven NMR data showed signals for 21 carbons, and DEPT spectral biosynthetically in vivo as well as in vitro, and it was data indicated the presence of 8 methyl groups, 4 methine carbons (2 bearing oxygen), 4 quaternary olefinic carbons, proposed to proceed as a [σ2a + π2a] photochemical re- 3 1 arrangement.6 Nevertheless, it has been suggested that the 1 carbonyl, and 4 sp quaternary carbons. The H NMR bicyclic core may also originate from its corresponding spectrum showed the following 8 methyl group signals: 1 11 methoxy group (δ 3.98), 2 olefinic methyls (δ 1.89 and δ acyclic precursor either by thermal [π4a + π2a] or intramo- 12 1.93), 4 methyls on quaternary carbon (δ 1.03, δ 1.20, δ lecular photochemical [π4s + π2a] Diels-Alder reactions. 1.38, δ 1.55), and a secondary methyl group (δ 1.19 d, J ) 7.5 Hz). Additional signals at δ 2.43 (q, J ) 7.5 Hz) for a methine quartet (COSY coupled with a secondary methyl group) as well as three methines, two of them bearing oxygen (δ 2.39, s; δ 3.18, s), complete all the protons of 1. All C-H correlations for 1 were detected in the HSQC spectrum. The comparison of the chemical shifts of carbons C-1-C-5 and the H3-15, H3-16, and H3-21 methyls with those of the corresponding polypropionate congeners7 indi- cates the presence of an R-methoxy-γ-pyrone fragment a. Our interest in benthic organisms from both sides of the This fragment was confirmed by the following HMBC isthmus of Panama13-15 prompted us to study the Pacific correlations: H -15, H -16/CdO; H -15, H -21/C-1 and H - sacoglossan E. diomedea. In this paper, we report the 3 3 3 3 3 16/C-4, C-5. isolation of two complex polyketide derivatives elysiapyrone Considering that one carbon of the molecular formula A 1 and elysiapyrone B 2 containing a bicyclo[4.2.0]octane corresponds to a methoxy group, the remaining 20 carbons (2) Green, B. J.; Li, W.-Y.; Manhart, J. R.; Fox, T. C.; Summer, E. J.; account for a maximum of 6 propionate units, 2 of them Kennedy, R. A.; Pierce, S. K.; Rumpho, M. E. Plant Physiol. 2000, 124, forming a γ-pyrone ring. The IR spectrum suggested that 331-342. no oxygen of the molecular formula was a hydroxyl group (3) Pierce, S. K.; Massey, S. E.; Hanten, J. J.; Curtis, N. E. Biol. Bull. -1 2003, 204, 237-240. and revealed absorption for a dienone at 1548 cm. Since (4) Cimino, G.; Ghiselin, M. T. Chemoecology 1998, 8,51-60. the γ-pyrone ring contains all the unsaturations and 5 of the (5) Ireland, C.; Faulkner, D. J. Tetrahedron 1981, 37, 233-240. (6) Ireland, C.; Scheuer, P. J. Science 1979, 205, 922-923. residual 13 carbons are methyl groups, the molecule must (7) Gavagnin, M.; Spinella, A.; Castelluccio, F.; Cimino, G. J. Nat. Prod. be tricyclic and the characteristic cyclohexane side-chain - 1994, 57, 298 304. must form part of a ring. (8) Gavagnin, M.; Mollo, E.; Cimino, G.; Ortea, J. Tetrahedron Lett. 1996, 37, 4259-4262. In the absence of hydroxyls, the remaining two oxygens (9) Kay, P. S.; Faulkner, D. J. Bol. Soc. Chil. Quı´m. 1984, 29, 329- must be epoxidic, which is concordant with the 13C NMR 332. (10) Dawe, R. D.; Wright, J. L. C. Tetrahedron Lett. 1986, 27, 2559- chemical shifts for each methine bearing oxygen (δ 61.1 and 2562. 62.1). The vicinal bis-epoxide fragment c was confirmed by (11) Miller, A. K.; Traumer, D. Angew. Chem., Int. Ed. 2003, 42, 549- 552. a HMBC experiment, particularly by the long-range cor- (12) Moses, J. E.; Baldwin, J. E.; Marquez, R.; Adlington, R. M.; relations H3-18, H3-19/C-9; H-9/C-11 and H3-19/C-11. Claridge, T. D. W.; Odell, B. Org. Lett. 2003, 5, 661-663. The key to establishing the presence of a cyclobutane ring (13) Dorta, E.; Dı´az-Marrero, A. R.; Cueto, M.; D’Croz, L.; Mate´, J. L.; San-Martı´n, A.; Darias, J. Tetrahedron Lett. 2004, 45, 915-918. fragment b was the HMBC correlations of the H3-17 with (14) Dorta, E.; Dı´az-Marrero, A. R.; Cueto, M.; D’Croz, L.; Mate´, J. L.; C-6, C-7, and C-13 crossed with H3-14/C-6 and H3-20/C-7, Darias, J. Org. Lett. 2004, 6, 2229-2232. (15) Dorta, E.; Dıaz-Marrero, A. R.; Cueto, M.; D’Croz, L.; Mate´ J. L.; C-13. In addition to biogenetic considerations, the HMBC Darias, J. Tetrahedron Lett. 2004, 45, 7065-7068. correlations of C-5 with H-7, H-13, H3-17 established the 416 Org. Lett., Vol. 7, No. 3, 2005 regiochemistry of the γ-pyrone ring on fragment b. Finally, formation of the bicyclo[4.2.0]octane moiety with the aim fragments b and c were linked through the selected HMBC of justifying the unusual finding that endriandic acids occur correlations: H3-18/C-7; H3-20/C-11. This completes the in nature in racemic rather than enantiomeric form. Black’s planar structure of elysiapyrone A as depicted in 1. suggestive hypothesis postulated successive nonenzymatic 25 Elysiapyrone B, 2, colorless oil, [R]D )+225° (c 0.08, electrocyclization reactions (path a, Scheme 1) from a + CHCl3) HREIMS [M] 360.1925 (calcd for C21H28O5, 360.1936) possesses the same molecular formula, and similar IR spectrum and 1H NMR data, as 1, suggesting that the Scheme 1. Possible Biogenetic Pathways for 1 and 2 difference between both compounds must be stereochemical in nature. NOESY experiments, Figure 1, aided in establishing the Figure 1. Selected NOEs of 1 and 2. stereochemistry of the substituents on the bicyclo[4.2.0]- octane core of 1. NOESY experiments of 1 revealed a syn- periplanar relationship between three vicinal angular methyl polyunsaturated achiral substrate. This hypothesis was later groups (Me-14, Me-17, Me-20); thus, the six-membered rings experimentally supported by a biomimetic synthesis18,19 of are face-to-face on the opposite side of the cyclobutane ring.